1. Field of the Invention
This invention relates to a power or battery drain control device for a camera, which device makes it possible to supply electrical current to the light metering and exposure value display circuits in the camera by a prescribed necessary time.
2. Description of the Prior Art
The conventional use of a photo-conductive element in combination with a correction resistor and a galvanometer is usually sufficient to measure the intensity of light in a photographic camera. When the battery drain control switch is left on due to forgetfullness of the operator, therefore, the battery never suffers a rapid drain of its electrical energy, because the load on the battery is very small.
However, because the photo-conductive element does not have good light response characteristics including such linearity as may be desired to improve the accuracy of the light measurement, there is a recent trend to employ a silicon photo-cell in its place. This silicon photo-cell has a characteristic feature which can be utilized to its full extend when in a zero-bias drive condition, or to logarithmically compress the intensity of the light sensed. For this purpose, use is made of an operational amplifier device. This, however, greatly increases the rate of consumption of electrical energy from the battery. Thus, it is very possible that when the battery is allowed to remain connected to the photo-cell, it will be drained of its electrical energy entirely in a few hours, or a few days.
In order to prevent accidental wasteful consumption of the electrical energy of the battery, the following three methods have been proposed. According to the first proposal, the power supply control switch is arranged to be opened when the film winding lever is retracted from its working position. According to the second method, the switch is arranged to cooperate with the shutter release button. According to the third method, the battery is associated with a timer responsive to actuation of the release mechanism of the camera for controlling the period of energization of the light metering circuit and the like.
However, these conventional methods have many disadvantages. In the first method, when a sequence shooting is repeated frequently, the retracting movement of the film winding lever above the camera body followed by advancement of the lever to the working position upon each exposure is very troublesome to perform. Therefore, the operator lets the lever remain at the working position. Then, the resultant wasteful consumption of electrical energy amounts to a considerable level. Further, there is some possibility of the occurrence of the camera being put away with the lever advanced, with the result that the life of the battery ends soon afterwards.
In the second method, the removal of the operator's finger from depressing the release button is assuredly followed by the cutoff of the power supply, so that an advantage is provided in that the necessary manipulation of the power supply control switch cannot be overlooked. But, when both of the operator's hands are placed on other operating members, for example, the distance adjusting ring in the left hand and the grasp rod of the tripod in the right hand to obtain desired framing, it is impossible to make use of the exposure information at the same time.
The third method has a disadvantage arising from the fact that each time the release button is more or less actuated, the timer is rendered operative. After the exposure value is observed by looking through the finder, the once half-depressed release button may be returned so that no exposure follows. In this case, the timer operates also with the supply of power for no purpose. Thus, a wasteful consumption of electrical energy results.
Another disadvantage is that since the timer starts to operate at the time of the initial depression of the button, on the assumption that the prescribed timing period is 15 seconds, when the release button is allowed to return after a 10 second period of depression, the remaining period of the power supply is 5 seconds so that there is no longer sufficient time available, particularly when the photographer uses his hands in adjusting the other members of the camera, as has been mentioned above. Thus, the photographer is hampered from making an exposure at the proper time, and will be unable to obtain the benefits of all the features of the camera.